This invention relates to precision comparator circuits which may be driven from a single power supply having inputs common moded to a ground reference potential and, more particularly, to integrated comparator circuits that include level shifting circuits coupled between the inputs and output thereof and circuit means for reducing the bias currents appearing at an input thereof.
Differential comparator circuits which are suited for manufacture in monolithic integrated circuit form are well known in the prior art. One such type of comparator circuit comprises a pair of PNP transistors the bases of which are the differential inputs thereof. The collectors of the PNP transistors may be connected to ground reference to provide good noise immunity. In this type of structure, a voltage level shift circuit is required to be coupled between the emitters of the PNP transistors and the output of the comparator in order to provide sufficient input voltage output swing. Typically, the voltage level shift circuit includes a diode connected NPN transistor connected in a series-conduction path to the emitter of one of the PNP transistors and a NPN transistor having its collector-emitter connected in a series-conduction path to the emitter of the second one of PNP transistors and to an output of the comparator circuit. The base of the NPN transistor is connected to the interconnected base and collector of the diode connected transistor.
The above described comparator circuit produces an undersirable current flow out of the input of the comparator connected to the second PNP transistor whenever the differential voltage applied across the inputs of the comparator circuit is such to cause the second PNP transistor to be rendered conductive. This current flow is undesirable because it flows directly to the driving source connected to the base of the second PNP transistor through the source impedance producing an IR voltage drop thereacross which is often undesirable.
Thus, there exists a need for providing a means for reducing the bias current flowing in the input of a precision comparator of the type described above.